fUGHT International, 12 January 1967 49
Two-and-a-half Engines?
BY R. A. TWOMEY*
THE COURSE OF HISTORY has always been most difficult toassess from close quarters, and this is as true of thedevelopment of the air transport industry as of any other
human endeavour. In this area we may well wonder what
trends of opinion and what technical contributions will even-
tually be ascribed to the year 1966. Little doubt it will be
remembered for the acceptance of the long-range jumbo jet,
but what of the trends in engine configuration in the short/
medium-haul field? In the recent crop of large-capacity airbus
designs, the HBN.100, Sud Galion, Lockheed Ten-Eleven,
BAC "Two-Eleven/VC5," and new Boeing and Douglas short-
haul projects, are we witnessing a return to the twin-engined
era?
This is a question which greatly concerned the delegates
to the recent BALPA airbus symposium (Flight for Novem-
ber 10 and 17, 1966). The attractions of the advanced tech-
nology high-bypass-ratio engines to the aircraft manufacturer
and operator are obvious and logical. Such engines offer a
specific fuel consumption dramatically better than the engines
they will supersede; they also produce phenomenal power, and
so invite a reduction in the number of power units required.
If, therefore, these new aircraft are indeed to be twins, their
operators will have the benefit of reduced capital and main-
tenance costs, and smaller spares holdings (though probably
more spare engine positioning in view of the unacceptability,
in this case, of engine-out ferry flights back to base). It is
also undeniable that engine compatibility with the Boeing 747
(JT9D) offers considerable advantages.
The fact is that the thrust of two JT9Ds (2 X 41,0001b) is
ideal for an aircraft expected to weigh around 250,0001b.
A 210,0001b take-off weight for a 350 n.m. stage has been
quoted for the HBN.100, and 276,0001b gross for the longer-
runway, longer-range Lockheed Ten-Eleven. One could say
that the die has been cast already.
The pilots, on the other hand, are less than satisfied with the
prospect of supporting 250 passengers on a wing and a pair.
The four-engined configuration, which grew out of engine-
stretch limitations to become the standard in piston-engined
days, is still the average pilots' ideal. But undoubtedly the
greater reliability of the gas turbine has made the three-
engined solution equally acceptable, particularly when applied
to short-haul rather than trans-oceanic work.
However, because of the sophistication of modern aircraft
and the extension of the task they are called upon to perform,
the argument is as much one of systems as of motive power.
The airbus is seen as a vehicle which by its very nature must
be capable of operation to at least Category 3A limits (zero
cloudbase, 700ft runway visual range), and this even after
an engine failure. In addition, the requirement is for an in-
dependent emergency electrical supply, a third hydraulic sys-
tem, and pressurisation and anti-icing services which are
usable throughout take-off, landing and overshoot without
performance penalties.
If this reads like a minimum requirement for three engines
*The author is a BEA pilot but the ideas put forward in thisarticle are not necessarily those of BEA or BALPA to both of
which the "2-J- engine" suggestion was put early in 1966—Ed.
for future large short-haul aircraft, I fear we are heading
for disappointment. The adoption of the high-bypass-ratao
engine offers tremendous advantages and is therefore inevitable;
choice of a forward-fan layout appears to preclude installa-
tion on the aircraft's centreline. In any event, much though
we would like it, operationally, the installation of three JT9Ds
would be a financially unacceptable way of providing the
performance and the triplicated services required.
The answer proposed so far has been the use of an auxiliary
power unit cleared to run in the air, and this is a most wel-
come step in the right direction in that it provides airborne
systems back-up for the emergency case. However, one would
probably need two APUs of current type to cope with the
ground electrical and air-conditioning loads of the large air-
craft we are considering, and thus must inevitably lead to
a search for an APU of an altogether greater capacity. Why
not, therefore, use an existing prime mover, fully developed
and with R&D costs all amortized, as the third engine-cum-
APU? The Spey 506, with a useful 10,0001b thrust, might well
fill the bill perfectly; it is even fair to say that for one operator
at least it has already performed invaluable service as an
APU stand-in.
Consider the advantages of powering the large short-haul
aircraft of the next decade with two large engines and one
smaller one. If we use two JT9Ds and one Spey 506 we have
all the benefits of advanced technological engineering, plus
systems redundancy to the standard required for all-weather
operations. Available take-off power is raised nearly ten
per cent, offering the possibility of considerable stage-length
improvement (as in the Trident 3B with its proposed 6,0001b
booster jet), and further abatement of noise. Alternatively,
we have the ability to use less than full JT9D power on take-
off, resulting in more rapid growth of TBO life. Mr Stephen
Wheatcroft, in his BALPA symposium paper, pointed to
two European airbus range requirements—one of approxi-
mately 400 n.m., the other of more than twice that distance.
A flexible take-off power operational philosophy is ideally
suited to such a vehicle.
After take-off or at top of climb, as required, the small
engine would normally be reduced to idling r.p.m., or even
shut down if 100 per cent relight success could be guaranteed.
On initial approach it would be brought in again at r.p.m.
high enough to provide the back-up electrical power supply,
to facilitate the operation of auto-throttle on the two main
engines, and to be available in case of overshoot.
There are other advantages. The bogy of first-segment
zero climb gradient is disposed of, as this is not applicable
to a three-engined aircraft. Add to this the consoling thought
that after a critical engine failure at Vi, instead of losing
50 per cent power, as in the case of a twin, we would lose
only 45 per cent, or effectively less, if "contingency" pro-
cedures were applied.
Existing smaller and less expensive aircraft are doubtless
very well able to fulfil their design task with two, or three, or
four engines. But (if it is permissible to misquote one large
operator's erstwhile slogan), in the airbus era there are
powerful arguments for providing feweT than three engines,
but more than two!
BWIA Joins IATA On lanuary 1 British West Indian Air-
ways became an active member of IATA, bringing the total
membership to 87, with 14 associate members.
Voice-recorder Delay The FAA has once again extended
the deadline for the installation of cockpit voice recorders in
large passenger aircraft. The deadline-^reviously December 1,
1966, for jets and lanuary 1, 1967, for pressurised piston air-
craft—is now March 1, 1967, for both. The date for jets has
twice before been postponed because of technical problems in
the recording equipment; installation difficulties have also been
met in the development of equipment for piston aircraft. The
FAA regulation was originally issued in September 1964.
SAA to New York South African Airways plans to start
operating to South and North America in 1968. The services
will probably be operated with stops only at Rio de Janeiro
(4,600 miles from Johannesburg) and at Bermuda for refuelling.